Further research into genetic links has been conducted by Sherrington, in the form of gene-mapping studies. Sherrington found that there is a gene located on chromosome 5 that has been linked to schizophrenia in a small number of extended families where a number of family members had the disorder. However, further research into the gene on chromosome 5 has failed to replicate Sherrington’s findings and so reliability is low. Kennedy suggested that the validity of the research is questionable as the findings may be only representative of the original sample.
Gottesman conducted further research into the concordance rates for monozygotic and dizygotic twins. Gottesman’s analysis of twin studies found that there was a 48% concordance for monozygotic twins and only 17% for dizygotic twins. Gottesman also reported that the concordance rate for identical twins brought up apart was very similar to that for identical twins brought up together. Therefore this suggests that the high concordance rate for identical twins is not due to them being treated in a very similar way within the family i.e. nature over nurture. As the monozygotic twins are genetically identical whilst dizygotic twins are no more alike than ordinary siblings, this supports the involvement of genetic factors. If these were not important, and instead nurture was the key determinant, then there should be no difference in the concordance rates between monozygotic and dizygotic twins. But, because there isn’t 100% concordance schizophrenia cannot be a wholly genetic disorder.
However, Kamin argued that the twins did not spend all of their time apart during childhood. Some were raised by close relatives and went to the same school, so they did share a similar environment. In addition, Loehlin and Nichols stated that the higher concordance rate in monozygotic twins may be due to the fact that monozygotic twins tend to be treated more similarly than dizygotic twins and so nurture may explain the concordance rates rather than nature.
One of the problems of any comparison of concordance rates between people in the same family is that they also share a similar environment. To eliminate this factor adoption studies have been used, in which children who have been fostered or adopted are compared with their biological and adoptive relatives in terms of rates of schizophrenia. Tienari’s study showed that 10.3% of adopted children who had a schizophrenic mother developed schizophrenia in adulthood, compared with only 1.1% of adopted children who did not have a schizophrenic mother. This supports the importance of genetic factors and is evidence against the role of nurture. Further research into adoption studies was conducted by Hetson, who compared 47 children of schizophrenic mothers who had been adopted or fostered during the first month of their life. A control group was used of 50 children who has been raised in the same homes as these children. Interestingly, none of the control group developed schizophrenia but 16.6% of the children of schizophrenics did. Furthermore, these 47 children were far more likely to have been diagnosed with other psychological abnormalities and be involved in criminal activities than were the control group.
Due to the fact that there isn’t 100% concordance rates of schizophrenia in any of the gene studies there must be other factors that affect the likelihood of developing the disease. However, it can be said a predisposition is inherited. Factors such as chemicals in the brain, brain receptors, Fromm – Reichmann’s ‘schizophrenogenic mother hypothesis’, culture and social class all play a part in the likelihood of developing schizophrenia.